1. Field of the Invention
This invention relates to spark plugs in general and more particularly to spark plugs for use in internal combustion engines.
2. Background of the Invention
A conventional spark plug includes a metallic shell adapted to be fitted into an opening of an engine wherein an air-fuel mixture is present. This area is typically referred to as a cylinder or combustion chamber within the engine. The shell of the spark plug accommodates a ceramic or other insulating structure through which an electrode extends into the combustion chamber. One end of the electrode is connected to an ignition system that supplies an high energy signal to the spark plug, and the other end of the electrode terminates within the combustion chamber. The spark plug provides an electrical arc or spark required to initiate combustion of the air-fuel mixture within the combustion chamber. A ground electrode (typically a projection or protrusion extending inward from the shell of the spark plug) is disposed in spaced apart relation with the electrode and provides a gap across which a high energy arc is established via the ignition system of the engine. The ground electrode or protrusion is mechanically displaced so that a predetermined distance or air gap is established between the center electrode and the ground electrode.
In systems well-known in the art, the spark plug of an internal combustion engine includes a predetermined spark gap or air gap which is mechanically adjusted prior to installation of the spark plug into a corresponding receptacle of the engine. Normally, the spark gap is adjusted to a length between 0.025 inches and 0.060 inches to provide an arc or spark having desired characteristics necessary for initiating proper combustion of the air-fuel mixture. When the engine is cold, it is more difficult to generate a spark between the electrode and the ground projection than is the case when the engine is hot. Further, it is well known that high load conditions require a small spark gap, where as low load conditions require a larger spark gap for proper combustion of the air-fuel mixture to take place.
3. Description of the Prior Art
Pratt, Jr., U.S. Pat. No. 3,974,412, discloses a spark plug wherein the path and consequently the length of the arc discharge is varied by virtue of the repulsion of two oppositely directed electric currents. The result is an arc whose length is much longer than ordinarily obtainable. Varying the current supplied to the arc results in a radial force useful in moving the arc in a radial direction with respect to the electrode and ground potential structures.
Dibert, U.S. Pat. No. 4,906,889, discloses a spark plug having an electrode which is grooved to enlarge its area and enable a ground projection or wire to react like a bimetallic element in response to changes in combustion chamber temperatures to vary the length of the spark gap.
Pratt, Jr., U.S. Pat. No. 4,087,719, discloses a spark plug wherein corona discharge is employed to create a long arc and to determine the path of the arc. Electrode and ground potential surfaces are oriented so that a radial force is provided to the arc to encourage the arc to grow or increase its length. The arc created is generally parallel with the electrode of the spark plug.
Almquist et al., U.S. Pat. No. 4,046,127, discloses a spark plug structure wherein engine vacuum levels or engine temperature provide a basis for adjusting the length of a spark or arc. The arc is varied in length between two electrodes by a third electrode situated near the two electrodes and movable with respect thereto. The third electrode is displaced or moved to mechanically shorten or lengthen the spark gap according to sensed temperature or vacuum levels.
Dingman, U.S. Pat. No. 3,219,866, discloses a spark plug structure having diverging gap electrodes disposed between two magnetic pole pieces to produce a directional advance of an arc therebetween.
Tozzi, U.S. Pat. Nos. 4,471,732 and 4,760,820, disclose a plasma jet plug structure including a plasma medium for generating a plasma and discharging the plasma as a jet into the combustion chamber of an internal combustion engine under the accelerating influence of a magnetic field.
Tozzi, U.S. Pat. No. 4,766,855, discloses a plasma jet plug structure similar to 4,471,732 and 4,760,820, and further includes an orifice for accelerating the plasma out of the plug cavity, under the influence of an accelerating magnetic field, with a ring vortex structure.
It has been determined that low ignition density conditions require a larger arc to provide sufficient ignition energy (approximately 17 milliJoules) to be discharged across the gap and promote proper combustion. Conversely, high ignition densities require smaller arcs (or arcs having a lower energy requirement). Therefore, less energy should be discharged into the spark gap under high ignition density conditions to avoid excessive voltages (in excess of 30,000 volts) which may be in excess of the dielectric capability of the high voltage harness of the internal combustion engine.
Thus, a spark plug device including a variable length spark gap that results in improved combustion stability at low loads and yet provides a small spark gap for high loads in the operation of a lean burn internal combustion engine is needed.